The invention relates to a thin film battery and its method of manufacture.
A thin film battery typically comprises a substrate having one or more thin films thereon, which may serve as, for example, current collectors, a cathode, an anode, and an electrolyte, that cooperate to store electrical charge to generate a voltage. The thin film batteries typically are less than about 1/100th of the thickness of conventional batteries. The thin films are typically formed by thin film fabrication processes, such as for example, physical or chemical vapor deposition methods (PVD or CVD), oxidation, nitridation or electro-plating. The substrate material is selected to provide good dielectric properties and good mechanical strength. Suitable substrate materials may include for example, oxides such as aluminum oxide and silicon dioxide; metals such as titanium and stainless steel; and semiconductors such as silicon.
However, conventional substrate materials often limit the ability of the battery to store electrical energy to achieve high energy density or specific energy levels. The energy density level is energy level per unit volume of the battery. The specific energy level is the energy level per unit weight of the battery. Conventional batteries typically achieve energy density levels of 200 to 350 Whr/L and specific energy levels of 30 to 120 Whr/L. However, it is desirable to have a thin film battery that provides higher energy density and specific energy levels to provide more power per unit weight or volume.
The ability to achieve higher energy levels is also enhanced by forming a crystalline cathode film on the substrate. The crystalline cathode film can also provide better charging and discharging rates. However, it is difficult to fabricate thin film batteries having crystalline cathode films on the substrate. Typically, the cathode is a thin film deposited on the substrate in the amorphous or microcrystalline form, and thereafter, crystallized by annealing at high temperatures. For example, an amorphous or microcrystalline film of LiCoO2 is typically annealed at about 700° C. to obtain a crystalline LiCoO2 cathode film. However, the higher annealing temperature constrains the types of materials that may be used to form the other thin films on the substrate. The other thin film materials should not, for example, soften, melt, oxidize, or inter-diffuse at annealing temperatures. The annealing process may also generate thermal stresses that arise from the difference in thermal expansion coefficient of the substrate, cathode, and current collector, resulting in delamination or peeling off of the thin films or even the entire thin film battery structure. Thus, conventional methods are often deficient in their ability to fabricate the crystalline cathode film of the thin film battery.
Thus it is desirable to have a thin film battery capable of providing relatively high energy density and specific energy levels. It is also desirable to reduce the temperatures of fabrication of the crystalline thin film materials, especially in the fabrication of cathode comprising LiCoO2.